In internal combustion engines for vehicles, it is sometimes desirable to have the facility for switching between different operating modes. For example, it is possible to switch between a conventional symmetrical cycle and an asymmetrical cycle, such as a so-called Miller cycle, by varying the timing of the inlet valve closure during the engine's induction stroke. The advantage in being able to switch between these different operating modes lies, for example, in the ability to vary the effective compression ratio of the engine in order to optimize efficiency and reduce exhaust emissions to a minimum. For this purpose, therefore, variable valve actuation is necessary.
The experimental use of electronically controlled hydraulic actuators for variable valve actuation as an alternative to mechanical valve systems is already known. These known systems are at present still very expensive and still not sufficiently reliable and robust and require a very sophisticated system of timing in order, among other things, to avoid collision between valves and piston and to cope with a viscosity that varies with temperature.
Other mechanical systems are disclosed, for example by U.S. Pat. No. 4,829,949, which permit mechanical change-over from one operating mode to another. These known systems are mechanically complex and call for extremely high precision both in manufacturing and servicing.
One known method is to achieve variable valve movements by means of mechanical-hydraulic systems of the “lost motion” type. These are characterized by a hydraulic link which forms part of the mechanical valve mechanism and which can be alternately opened or closed by means of a solenoid valve, for example. When closed, the movement of the valve is controlled by the cam curve in the same way as in a conventional mechanical system. When valve closure is to be advanced, for example, the hydraulic link is opened by means of the solenoid valve, so that the valve ceases to follow the cam curve and can thereby be made to close.
The disadvantage with the system described above is that the energy stored in the valve spring (which in a conventional mechanical system is largely recovered as the valve falls due to the fact that the valve spring force imparts a driving torque to the camshaft) is for the most part lost in a system of the “lost motion” type when the engine is operating in Miller cycle mode.
For engine designs in which the Miller cycle represents the most commonly occurring mode of operation, this energy loss may have a significant impact on the fuel consumption of the engine during a collective drive test cycle.